1. Field of the Invention
The present invention relates to the activation of an inductance loop vehicle detector that senses vehicles, such as vehicles located at an intersection for the purpose of triggering a traffic signal device. Various examples of the invention are particularly related to a magnetic device, or a method that uses a magnetic device, attached to a vehicle and which assists in activating an inductance loop vehicle detector.
2. Description of Background Art
In order to regulate vehicular traffic in an efficient manner, intersections with traffic signals often include an inductance loop vehicle detector. This type of detector senses the presence of vehicles located at an intersection. The basic configuration of such a vehicle detector includes an induction loop (e.g., a wire coil) that is located beneath a roadway. The induction loop is connected to a control box that includes one or more oscillators and an oscillation counter. The oscillator produces an oscillating signal in the induction loop and the oscillation counter stores information relating to the frequency of the oscillation in the induction loop.
When a vehicle approaches such a vehicle detector, conducting material (e.g. metal) in the vehicle decreases the inductance of the induction loop, thereby increasing the oscillation frequency of the detector. The decrease in inductance occurs because the conducting material in the vehicle acts as a shortened turn in the induction loop. To determine whether a vehicle is present at the intersection, the vehicle detector compares the current oscillation frequency with a previously measured oscillation frequency. If the current oscillation frequency is greater than the prior oscillation frequency, it determines that a vehicle is present. If, however, the current oscillation frequency is less than or equal to the prior oscillation frequency, then the detector determines that a vehicle is not present.
As previously noted, this type of vehicle detector may be deployed at a traffic intersection to alter a traffic signal controller of the presence of a vehicle. With this arrangement, a vehicle may enter an intersection, remain at an intersection, or leave an intersection between successive oscillation counts. If the oscillation counter senses an increase in the oscillation frequency, the detector generates a control signal alerting a traffic signal controller of the presence of a vehicle. Generally, the traffic signal controller will respond to the control signal after a preset delay period by changing the traffic signal configuration (e.g., from a red traffic light signal to a green traffic light signal). If the traffic signal controller has not altered the traffic signal configuration and a succeeding oscillation count remains at the higher level, the detector generates a control signal to alert the traffic signal controller of the continued presence of a vehicle. Similarly, if a succeeding oscillation count returns to a base level, the detector may generate a control signal to alert the traffic signal controller of the vehicle's departure, thereby eliminating the need for a change in the traffic signal's configuration.
In addition to vehicles that are located proximal to the inductance loop, environmental factors may affect the inductance of the loop. In multi-lane intersections, for example, vehicles stopped in one lane may alter the inductance of a loop located in a neighboring lane. Where intersections are located near train tracks, the presence of a train may also affect loop inductance. Further, fluctuations in the electrical load of the detector may alter the inductance properties of the loop. Accordingly, inductance loop vehicle detectors are often calibrated such that small changes in oscillation frequency do not generate a control signal to traffic signal controller. Instead, only changes in frequency that are above the preset level will affect the operation of the traffic signal controller.
A common, detrimental effect of the calibration is that vehicles with a relatively small quantity of conducting material, such as bicycles, motorcycles, and smaller automobiles, do not create an inductance in the loop sufficient to alter the oscillation frequency of commonly used vehicle detectors beyond their calibrated level. When such a vehicle enters an intersection with an inductance loop detector, the detector does not generate a control signal to alert the traffic signal controller of the vehicle's presence. Accordingly, there is a need in the art for an apparatus and/or method that may be used in conjunction with vehicles that have a relatively small quantity of conducting material to activate inductance loop vehicle detectors.